Tax exemption for a special catedory users

Tax exemption for a special catedory users

I have a category of users who are eligible to be exempted from paying tax. Is there any possibility to do it, so this category users not pay tax? It could be a field in registration, but fields of registration are not customizable.
Any advice how can I solve this issue will be much appreciated.

Aw: Tax exemption for a special catedory users

http://www.webdesigner-profi.de/joomla-webdesign/shop/addons/tax-client--usergroup.html